There are two ways to restart Chromecast. Just follow these steps:
From the Google Home app
1. Make sure your mobile device or tablet is connected to the same Wi-Fi as your Chromecast device.
2. Open the Google Home app
3. In the upper right corner of the home screen, tap Devices
to see your available Chromecast devices.
4. In the top right corner of the device you'd like to reboot, tap the device card menu
5. In the top right corner of the Device Settings page, tap the More button
From the power source
1. Unplug the power cord from the Chromecast device.
2. Leave it unplugged for ~1 minute.
3. Plug the power cord back in.